arm64 boot requirements

[Carl van Schaik]

Hi,

On 1 December 2015 Ard Biesheuvel <ard.biesheuvel at linaro.org> wote:
> On 1 December 2015 at 12:02, Mark Rutland <mark.rutland at arm.com>
> wrote:
> > Hi,
> >
> > On Mon, Nov 30, 2015 at 12:45:18PM +1100, Carl van Schaik wrote:
> >> In commit bd00cd5f8c8c3c282bb1e1eac6a6679a4f808091, the
> idmap_pg_dir
> >> and swapper_pg_dir where moved from before the kernel to after it.
> >>
> >> The problem is that these symbols fall outside the range covered by
> >> the ELF file - outside of any section.
> >>
> >> A bootloader which loads the kernel ELF file and dynamically
> >> determines where to place the DTB, may try place it after the
> >> kernel. We've just run into this problem and the DTB gets
> >> overwritten as soon as the pagetables are created.
> 
> Could you explain why you are using the ELF file and not the binary image
> file?
> This is not future proof: currently, the Image is a straight binary
> objcopy of vmlinux, but that is not guaranteed to remain that way.
> Things like KASLR may require post build steps that mangle vmlinux or
> Image afterwards.

The reason we've been using ELF files is mostly to do with legacy virtualization
related reasons in our systems, we used to patch symbols in the ELFs for example
pre device-tree. However, since it hadn't caused problems until now we had
continued to use it. We haven't yet added Aarch64 Linux boot image header parsing
but it should be trivial.

The other area we are looking into is optimized multi-VM static boot images by
constructing hypervisor-bundle images containing de-duplicated Linux sections,
allowing an ELF bootloader to populate multiple Linux VMs from a smaller boot
image - resulting in faster boot.

> >
> > We had similar issues with the BSS when booting Image files prior to
> > this and commit a2c1d73b94ed49f5 ("arm64: Update the Image header").
> > Since then, the image_size field in the Image header tells you how much
> > memory the kernel may clobber (including the BSS and page tables).
> >
> > Prior to that, the page tables were below the kernel, and also not
> > described in any ELF section.
> >
> > Others booting the kernel vmlinux haven't reported similar issues, so I
> > assume that either they are parsing the Image header, or getting lucky.
> > Parsing the header is necessary to get the correct text offset, too...
> >
> > Pratyush, Geoff, I understood you were loading the kernel vmlinux for
> > kexec. Do you parse the Image header to figure out where to place
> > things?
> >
> >> I'd suggest that the kernel either:
> >>  A. document this boot requirement for where not to load a DTB
> >
> > Do you have any particular suggestion?
> >
> > We already describe the Image footprint (including BSS and page tables)
> > by the image_size in the Image header, which is sufficient. The size of
> > the BSS and page tables is effectively unbound, so we can't define some
> > upper bound that will always be true.
> >
> > The documentation is written on the assumption that an Image file is
> > being used rather than a vmlinux. Perhaps that is something to consider.
> >
> >>  B. update the vmlinux.lds.S such that these symbols (and _end) are
> >> properly covered by a section in the ELF, and thus preventing this
> >> issue.
> >
> > I'm worried that this only solves this one case, and it means that there
> > are two (potentially conflicting) sources of information that a
> > bootloader might be using -- the ELF or the Image header. I don't want
> > to have to duplicate text_offset and so on, which implies that parsing
> > the Image header is necessary anyway.
> >
> > That's something we can discuss if you send a patch (inline rather than
> > attached).
> >
> 
> I think updating the linker script to put the page tables into a
> .pgdir section is reasonable, since it is part of the static footprint
> of the kernel.

I agree

> However, I strongly feel that the Image header should remain the
> authoritative source of information regarding the nature (big/little
> endian, page size) and the static footprint of the Image .

Agreed, and there are other ways to de-duplicate which will still work
with binary image inputs.